Semiconductor device with treated interfacial layer on silicon germanium

ABSTRACT

A method includes following steps. A silicon germanium layer is formed on a substrate. A surface layer of the silicon germanium layer is oxidized to form an interfacial layer comprising silicon oxide and germanium oxide. The interfacial layer is nitridated. A metal gate structure is formed over the nitridated interfacial layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 16/853,602, filed Apr. 20, 2020, now U.S. Pat. No.11,031,508, issued Jun. 8, 2021, which is a continuation application ofU.S. patent application Ser. No. 15/919,070, filed Mar. 12, 2018, nowU.S. Pat. No. 10,629,749, issued Apr. 21, 2020, which claims priority toU.S. Provisional Application Ser. No. 62/593,004, filed Nov. 30, 2017,which are herein incorporated by reference in their entirety.

BACKGROUND

Intentionally grown interfacial layer (IL) is used in order to arrange agood interface between the channel region and the gate insulator,especially with high-k dielectrics (e.g. HfO₂, HfSiO₄, ZrO₂, ZrSiO₄,etc.), and to suppress the mobility degradation of the channel carrierof metal-oxide-semiconductor field-effect transistors (MOSFETs).

However, when the channel region contains silicon germanium, theformation of IL very often results in dangling bond on the surface ofIL. The dangling bond decreases electron mobility at the channel region.One way to remove the dangling bond is to epitaxially grow a cap layeron the channel region. An addition of the cap layer increases thethickness of the channel region, and device dimension has to compromise.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a flow chart illustrating a method of fabricating asemiconductor device in accordance with some embodiments of the instantdisclosure;

FIGS. 2 through 18 are cross-sectional views of a portion of asemiconductor device at various stages in a replacement gate stackformation process in accordance with some embodiments of the instantdisclosure; and

FIGS. 19A through 19D are cross-sectional views of a portion of asemiconductor device in an interfacial layer treatment process inaccordance with some embodiments of the instant disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The fins may be patterned by any suitable method. For example, the finsmay be patterned using one or more photolithography processes, includingdouble-patterning or multi-patterning processes. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing patterns to be created that have,for example, pitches smaller than what is otherwise obtainable using asingle, direct photolithography process. For example, in one embodiment,a sacrificial layer is formed over a substrate and patterned using aphotolithography process. Spacers are formed alongside the patternedsacrificial layer using a self-aligned process. The sacrificial layer isthen removed, and the remaining spacers may then be used to pattern thefins.

A metal-oxide-semiconductor (MOS) device and a method of forming thesame are provided in accordance with various exemplary embodiments. Theintermediate stages of forming the MOS device are illustrated. Thevariations of the embodiments are discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements.

Referring to FIG. 1, a flow chart of a method 100 of fabricating asemiconductor device in accordance with some embodiments of the instantdisclosure is shown. The method begins with operation 110 in which achannel region is formed on a semiconductor substrate. The methodcontinues with operation 120 in which an interfacial layer is formed onthe channel region. Subsequently, operation 130 is performed. Theinterfacial layer is treated with trimethyl aluminum (TMA). The methodcontinues with operation 140 in which a high-k dielectric layer isformed on the interfacial layer after the treating the interfacial layerwith TMA. The method continues with operation 150 in which a gateelectrode is formed on the high-k dielectric layer. The discussion thatfollows illustrates embodiments of semiconductor devices that can befabricated according to the method 100 of FIG. 1. While method 100 isillustrated and described below as a series of acts or events, it willbe appreciated that the illustrated ordering of such acts or events arenot to be interpreted in a limiting sense. For example, some acts mayoccur in different orders and/or concurrently with other acts or eventsapart from those illustrated and/or described herein. In addition, notall illustrated acts may be required to implement one or more aspects orembodiments of the description herein. Further, one or more of the actsdepicted herein may be carried out in one or more separate acts and/orphases.

FIGS. 2 through 12 are cross-sectional views of intermediate stages information of a MOS device in accordance with some exemplary embodiments.Reference is made to FIG. 2. A wafer 10, which includes a semiconductorsubstrate 20, is provided. A silicon germanium (Si_(1-x)Ge_(x)) layer202 is formed on the semiconductor substrate 20, and the x rangesbetween about 0.15 and about 0.95. In the case in which x is smallerthan 0.15, the resulting silicon germanium layer has an amount ofgermanium that is too low to cause an adverse effect. In someembodiments, x may be higher than 0.95, and it indicates a high ratio ofGeO_(x) in the resulting silicon germanium. The Si_(1-x)Ge_(x) layer 202is epitaxially grown on the surface of the semiconductor substrate 20.Germanium has a higher lattice constant than silicon, and hence theresulting lattice structure of Si_(1-x)Ge_(x) layer 202 allows higherelectron hole mobility than the semiconductor substrate 20. Shallowtrench isolation (STI) regions (not shown) are formed in theSi_(1-x)Ge_(x) layer 202 and are used to define the active regions ofMOS devices.

Reference is still made to FIG. 2. A dummy gate stack 22 is formed overthe Si_(1-x)Ge_(x) layer 202. The dummy gate stack 22 includes a dummygate dielectric 24 and a dummy gate electrode 26. The dummy gatedielectric 24 includes silicon oxide in some exemplary embodiments. Inalternative embodiments, other materials, such as silicon nitride,silicon carbide, or the like, are also used. The dummy gate electrode 26may include polysilicon. In some embodiments, the dummy gate stack 22further includes a hard mask 28 over the dummy gate electrode 26. Thehard mask 28 may include silicon nitride, for example, while othermaterials, such as silicon carbide, silicon oxynitride, and the like,may also be used. In alternative embodiments, the hard mask 28 is notformed. The dummy gate stack 22 defines the channel region 32 in theSi_(1-x)Ge_(x) layer 202. The source and drain regions 38 (FIG. 3) arelater formed on opposing sides of the channel region 32.

Reference is still made to FIG. 2. Lightly-doped source and drain (LDD)regions 30 are formed, for example, by implanting a p-type impurity(such as boron and/or indium) into the Si_(1-x)Ge_(x) layer 202. Forexample, when the MOS device is a pMOS device, the LDD regions 30 arep-type regions. The dummy gate stack 22 acts as an implantation mask, sothat the edges of the LDD regions 30 are substantially aligned with theedges of the gate stack 22.

Reference is made to FIG. 3. Gate spacers 34 are formed on sidewalls ofthe dummy gate stack 22. In some embodiments, each of the gate spacers34 includes a silicon oxynitride layer and a silicon oxide layer. Inalternative embodiments, the gate spacers 34 include one or more layers,each including silicon oxide, silicon nitride, silicon oxynitride,and/or other dielectric materials. Formation methods of the gate spacers34 include but not limited to plasma enhanced chemical vapor deposition(PECVD), low-pressure chemical vapor deposition (LPCVD), sub-atmosphericchemical vapor deposition (SACVD), and other deposition methods.

Reference is still made to FIG. 3. Source and drain regions (referred toas source/drain regions hereinafter) 38 are formed in the Si_(1-x)Ge_(x)layer 202. In the embodiments in which the MOS device is a pMOS device,the source/drain regions 38 are of p-type. In some embodiments,source/drain stressors (also marked as 38) are formed in theSi_(1-x)Ge_(x) layer 202. The source/drain stressors form at least partsof the source/drain regions 38. FIG. 3 illustrates the embodiments inwhich the source/drain regions 38 fully overlap the respectivesource/drain stressors.

In the embodiments in which the MOS device is a pMOS device, thesource/drain stressors may include suitable dopant. The formation of thesource/drain stressors may be achieved by etching the Si_(1-x)Ge_(x)layer 202 and the semiconductor substrate 20 to form recesses therein,and then performing an epitaxy to grow the source/drain stressors in therecesses.

Reference is made to FIG. 4. A contact etch stop layer (CESL) 40 isformed over the gate stack 22 and the source/drain regions 38. In someembodiments, the CESL 40 includes silicon nitride, silicon carbide, orother dielectric materials. An interlayer dielectric (ILD) layer 42 isformed over the CESL 40. The ILD layer 42 is blanket formed to a heighthigher than the top surface of the dummy gate stack 22. The ILD 42 mayinclude flowable oxide formed using, for example, flowable chemicalvapor deposition (FCVD). The ILD layer 42 may also be a spin-on glassformed using spin-on coating. For example, the ILD layer 42 may includephospho-silicate glass (PSG), boro-silicate glass (BSG), boron-dopedphospho-silicate glass (BPSG), tetraethyl orthosilicate (TEOS) oxide,TiN, SiOC, or other low-k porous dielectric materials.

Reference is made to FIG. 5. FIG. 5 illustrates a planarization step,which is performed using, for example, chemical mechanical polish (CMP).The CMP is performed to remove excess portions of the ILD layer 42 andthe CESL 40. The excess portions over the top surface of the hard mask28 are removed. Accordingly, the dummy gate stack 22 is exposed. Inalternative embodiments, the hard mask 28 is removed during the CMP, inwhich the CMP stops on the top surface of the dummy gate electrode 26.

Reference is made to FIG. 6. Next, the dummy gate stack 22 is removed. Arecess 44 is formed as a result of the removal of the dummy gate stack22, in which the resulting structure is shown in FIG. 6. The removal ofthe dummy gate stack 22 exposes the underlying Si_(1-x)Ge_(x) layer 202.

FIGS. 7 through 12 illustrate formation of a replacement gate stack.Reference is made to FIG. 7. An interfacial layer 52 is formed over thechannel region 32. The interfacial layer 52 is used in order to arrangea good interface between the Si_(1-x)Ge_(x) layer 202 and the gateinsulator, especially with high-k dielectrics (e.g. HfO₂, HfSiO₄, ZrO₂,ZrSiO₄, etc.), and to suppress the mobility degradation of the channelcarrier of the metal-oxide-semiconductor field-effect transistors(MOSFETs). Chemical oxide prepared by diluted HF, Standard Clean 1(SC1), and Standard Clean 2 (SC2), plasma oxidation, ozonated deionizedwater treatment, rapid thermal oxidation (RTO), or the like may be usedto form the interfacial layer 52 in the replacement gate stack. Forexample, ozonated oxide can be grown by high ozone gas, either in gasphase or pre-dissolved in de-ionized (DI) water. In some embodiments,the interfacial layer 52 is in contact with the channel region 32.

After the oxidation of the Si_(1-x)Ge_(x) layer 202, a thin film ofinterfacial layer 52 is formed on the surface of the channel region 32.The interfacial layer 52 includes silicon oxide (SiO_(y), in which y islarger than 0) and germanium oxide (GeO_(y) in which y is larger than0). The proportion of germanium oxide in the interfacial layer 52resulting from the oxidation treatment is highly dependent on thegermanium content in the layer 202 over the semiconductor substrate 20.The higher is the germanium content in the layer 202, the higher is thegermanium oxide content of the interfacial layer 52. The lower is thegermanium content in the Si_(1-x)Ge_(x) layer 202, the lower is thegermanium oxide content of the interfacial layer 52. Germanium oxide isharmful to the quality of the interfacial layer formed on theSi_(1-x)Ge_(x) channel region. The harm to the channel region isevidenced by the increase in charged interface states. The harm to thechannel region is also evidenced by the decrease in mobility withincreasing amounts of germanium oxide in the interfacial layer.Accordingly, described herein are methods to scavenge or remove thegermanium oxide from the interfacial layer 52 including germanium oxideand silicon oxide that is formed on the Si_(1-x)Ge_(x) layer 202. Insome embodiments, the germanium oxide is substantially removed from theinterfacial layer 52, leaving silicon oxide remaining. In otherembodiments, after a scavenging step, the residual germanium oxide isless than 5%, for example.

Reference is made to FIG. 8. The removal of germanium oxide from theinterfacial layer 52 by the scavenging step is described below. Theunnumbered arrows show a thermal annealing treatment to the wafer 10. Afirst stage of the scavenging step is carried out by heating the wafer10 at a temperature of from about 500° C. to about 900° C. for about 1minute. If the heating temperature is lower than 500° C., the germaniumoxide cannot be completely removed from the inter facial layer 52. Ifthe heating temperature is higher than 900° C., source and draindegradation or interface roughness may occur. This thermal annealing isconducted in an atmosphere of from about 1 Torr to about 760 Torr. Thethermal annealing treatment is conducted in substantially oxygen-freecondition with inert gas, for example N₂ to prevent oxidation. Thescavenging step is effective because the germanium-oxygen bond ingermanium oxide is much weaker than both the silicon-oxygen bond insilicon oxide and the silicon-germanium bond in the Si_(1-x)Ge_(x) layer202. Accordingly, germanium oxide is easily removed, leaving siliconoxide remaining within the interfacial layer 52 a on the Si_(1-x)Ge_(x)layer 202. The first stage of the scavenging step removes a largeportion of the germanium oxide of the interfacial layer 52 before thewafer 10 is transferred to the atomic layer deposition (ALD) reactionchamber.

Reference is made to FIG. 9. The removal of germanium oxide from theinterfacial layer 52 a continues to a second stage. The unnumberedarrows show a trimethyl aluminum (TMA) pretreatment to the wafer 10in-situ. The TMA pretreatment is conducted in substantially oxygen-freecondition. After the first stage of germanium oxide scavenging step, thewafer 10 is transferred to an ALD reaction chamber (not shown),preparing for high-k dielectric layer deposition. Before the depositionof the high-k dielectric layer, the wafer 10 undergoes a TMApretreatment in the ALD reaction chamber. TMA is a strong reductant, andTMA precursor is provided for about 30 seconds. The TMA pretreatment iscarried out at a temperature of from about 150° C. to about 300° C. ThisTMA pretreatment is a consecutive process before the high-k dielectriclayer deposition by ALD, and the reaction conditions of the TMApretreatment is similar to the ALD reaction conditions of high-kdielectric layer deposition. The flow rate of TMA precursor is in arange from about 200 sccm to about 600 sccm. A flow rate lower than 200sccm may result in incomplete removal of the germanium oxide. Anatmosphere is chosen depending on the flow rate. In some embodiments,the atmosphere during TMA pretreatment ranges from about 1 Torr to about25 Torr. The remaining germanium oxide in the interfacial layer 52 a isthen removed by the TMA pretreatment. In some embodiments, the thermalannealing treatment and the TMA pretreatment are conducted in differentchambers.

Reference is made to FIGS. 19A through 19D, illustrating schematicdiagrams of germanium oxide scavenging step. As shown in FIG. 19A, asilicon germanium (Si_(1-x)Ge_(x)) layer 202 is formed. An interfaciallayer 52 is formed on the silicon germanium layer 202 by oxidation stepshown in FIG. 19B. The interfacial layer 52 includes silicon oxide andgermanium oxide, and the amount of silicon oxide and germanium oxidedepends on the silicon germanium ratio of the silicon germanium layer202. As shown in FIG. 19C, the unnumbered arrows show the first stage ofgermanium oxide scavenging in which thermal annealing treatment breaksgermanium and oxygen bonding so as to remove germanium oxide from theinterfacial layer 52. Silicon oxide remains as a component of theinterfacial layer 52 a. As shown in FIG. 19D, TMA pretreatment is thenperformed to remove remaining germanium oxide from the interfacial layer52 a. The TMA pretreatment is conducted in-situ of high-k dielectriclayer ALD process. The wafer 10 does not need to be transferred to adifferent chamber for TMA pretreatment which simplifies the fabricationprocess. The thickness of the interfacial layer 52 remains relativelyunchanged.

Reference is made to FIG. 10. In some embodiments, before the depositionof the high-k dielectric layer, an in-situ nitridation treatment isperformed. The nitridation treatment is performed in the ALD reactionchamber. The unnumbered arrows show the nitridation process. Germaniumoxide is removed by the two-stage scavenging step including thermalannealing and TMA pretreatment. The remaining silicon oxide of theinterfacial layer 52 b is converted into silicon oxynitride(SiO_(a)N_(b)) by the nitridation treatment with a nitrogen containingagent, in which a and b is larger than 0. The nitridation treatmentincludes, for example, NH₃ plasma in plasma enhanced ALD (PEALD) forabout 5 to 30 seconds, N₂ plasma in PEALD for about 5 to 30 seconds, orNH₃ gas annealing in ALD at about 300° C. to 500° C. for about 1 minute.If the duration of the plasma treatment is shorter than 5 seconds, theeffect of nitridation may be insufficient, resulting in germanium oxideformation in the subsequent process. If the duration of the plasmatreatment is longer than 30 seconds, the plasma intensity may damage theinterfacial layer 52 b. If the temperature of NH₃ gas annealing is lowerthan 300° C., the nitridation on the interfacial layer 52 b may notoccur, and the time duration allows sufficient silicon oxynitrideformation. This nitridation treatment further prevents the germanium ofthe Si_(1-x)Ge_(x) layer 202 out-diffusion. The interfacial layer 52 cis then a nitrogen-containing layer, e.g. a silicon oxynitride layer,that covers the channel region 32 of the Si_(1-x)Ge_(x) layer 202.

In some embodiments, the nitridation treatment may extend to aninterface between the interfacial layer 52 c and the Si_(1-x)Ge_(x)layer 202. This prevents the germanium of the Si_(1-x)Ge_(x) layer 202from out-diffusion. This also avoids the interfacial layer 52 c fromhaving an untreated portion. Such an untreated portion increases aneffective oxide thickness (EOT) of the gate stack, resulting in a lowgate control ability for a device. In some embodiments, the nitridatedinterfacial layer 52 c includes nitrogen therein, and the thickness ofthe nitridated interfacial layer 52 c is in a range from about 5 Å toabout 10 Å. If the thickness of the nitridated interfacial layer 52 c isless than about 5 Å, the nitridated interfacial layer 52 c may not bethick enough to prevent the germanium of the layer 202 fromout-diffusion, resulting in germanium oxide formation in the subsequentprocesses. On the other hand, if the nitridated interfacial layer 52 cis greater than about 10 Å, the EOT of the gate stack may be too thick,resulting also in a low gate control ability for the device.

In some embodiments, the temperature of the semiconductor substrate 20during the nitridation treatment is in a range from about 300° C. toabout 1000° C. If the temperature of the semiconductor substrate 20during the nitridation treatment is lower than about 300° C., the effectof nitridation may be insufficient, resulting in germanium oxideformation in the subsequent processes. If the temperature of thesemiconductor substrate 20 is greater than about 1000° C., thenitridation treatment may affect the underlying Si_(1-x)Ge_(x) layer202, resulting in the increase of the effective oxide thickness (EOT) ofthe gate stack, which causes a low gate control ability for a device.

In some embodiments, the plasma power of the nitridation treatment is ina range from about 50 w to about 650 w. If the plasma power of thenitridation treatment is lower than about 50 w, the effect ofnitridation may be insufficient, resulting in germanium oxide formationin the subsequent processes. If the plasma power of the nitridationtreatment is greater than about 650 w, the nitridation treatment mayaffect the underlying Si_(1-x)Ge_(x) layer 202, resulting in theincrease of the effective oxide thickness (EOT) of the gate stack, whichcauses a low gate control ability for a device.

Reference is made to FIG. 11. A high-k dielectric layer 54 is formed.The high-k dielectric layer 54 includes a high-k dielectric materialsuch as hafnium oxide, lanthanum oxide, aluminum oxide, or the like. Thedielectric constant (k-value) of the high-k dielectric material ishigher than 3.9, and may be higher than about 7, and sometimes as highas 21 or higher. The high-k dielectric layer 54 is overlying theinterfacial layer 52 c. The formation of the high-k dielectric layer 54is performed in the ALD reaction chamber. A work function metal layer 62is formed on the high-k dielectric layer 54. The work function metallayer 62 may include titanium aluminum (TiAl) in accordance with someembodiments. In some embodiments, a barrier layer (not shown) isinterposed between the work function metal layer 62 and the high-kdielectric layer 54. The barrier layer may include TiN, TaN, orcomposite thereof. For example, the barrier layer may include a TiNlayer (the lower part of barrier layer), and a TaN layer (the upper partof barrier layer) over the TiN layer.

Reference is still made to FIG. 11. In some embodiments, thesubsequently formed metal layers may include a block layer (not shown),a wetting layer (not shown), and a metal gate electrode 64. The blocklayer may include TiN, and the wetting layer may be a cobalt layer. Themetal gate electrode 64 may include tungsten, a tungsten alloy,aluminium, an aluminum alloy, or the like.

Reference is made to FIG. 12, illustrating a planarization step. Theplanarization step may be, for example, CMP for removing excess portionsof the high-k dielectric layer 54, work function metal layer 62, andmetal gate electrode 64 over the interlayer dielectric layer 42. Theinterfacial layer 52 c, high-k dielectric layer 54, work function metallayer 62 and metal gate electrode 64 form the replacement gate stack 72.

The replacement gate stack 72 has a nitrogenous interfacial layer 52 cinterposed between the high-k dielectric layer 54 and the Si_(1-x)Ge_(x)layer 202. The interfacial layer 52 c undergoes the thermal annealingand the TMA pretreatment and further to the nitridation process. Theseprocesses ensure germanium oxide desorption from the interfacial layer52 c and therefore maintains a lower interface state density (D_(it)) atthe interface between the interfacial layer 52 c and the Si_(1-x)Ge_(x)layer 202. A lower D_(it) is less likely to flatten on-off switch curveand allows higher electron mobility at the channel region. An epitaxialprocess to treat the Si_(1-x)Ge_(x) layer 202 surface can be omittedbecause the series of interfacial layer treatment minimizes danglingbonds thereon. Without the addition of an epitaxial cap on the channelregion, scaling of the channel body can be realized especially indevices like ultrathin body SiGe-OI (Silicon Germanium on Insulator)FET, FinFET, nano-wire FET and the like.

FIGS. 13 through 15 illustrate the formation of a replacement gate stackin some embodiments. Reference is made to FIG. 13, illustratingformation of a high-k passivation layer 82. After the dummy gate stack22 is removed and the recess 44 is created as described through FIGS.2-6, the interfacial layer 52 goes through a series of treatmentsincluding thermal annealing and in-situ TMA pretreatment as shown inFIGS. 7-9. The wafer 10 is in the ALD reaction chamber when the TMAprecursor is introduced to the reaction chamber prior to high-kdielectric layer deposition. After the annealing and TMA pretreatment,the germanium oxide is readily removed from the interfacial layer 52 b,leaving silicon oxide as the key component in the interfacial layer 52b. In some embodiments, the interfacial layer nitridation is omitted inthe process. Alternatively, a high-k passivation layer 82 is formed onthe interfacial layer 52 b that goes through thermal annealing and TMApretreatment.

The high-k passivation layer 82 is formed by ALD prior to high-kdielectric layer deposition in the same ALD reaction chamber. The high-kpassivation layer 82 conforms to the replacement gate recess 44, inwhich the sidewalls of the spacers 34 and the top surface of theinterfacial layer 52 b are covered up thereby. The high-k passivationlayer 82 reacts with the interfacial layer 52 b. Therefore, the high-kpassivation layer 82 includes, for example, high-k silicate, germanate,or combinations thereof in its bottom portion. The concentration of thehigh-k silicate or germanate in the high-k passivation layer 82decreases as a distance from the interfacial layer 52 b increases.Examples of high-k materials in the high-k passivation layer 82 may beAl₂O₃, La₂O₃, Y₃O₃, or combinations thereof. This high-k passivationlayer 82 prevents germanium of the Si_(1-x)Ge_(x) layer 202out-diffusion. A thickness of the high-k passivation layer 82 may rangebetween about 5 and 10 Å.

In some embodiments, the thickness of the high-k passivation layer 82 isin a range from about 5 Å to about 10 Å. If the thickness of the high-kpassivation layer 82 is less than about 5 Å, the high-k passivationlayer 82 may not be thick enough to prevent the germanium of theSi_(1-x)Ge_(x) layer 202 from out-diffusion, resulting in germaniumoxide formation in the subsequent processes. If the thickness of thehigh-k passivation layer 82 is greater than about 10 Å, the effectiveoxide thickness (EOT) of the gate stack may be too thick, resulting in alow gate control ability for a device.

Reference is made to FIG. 14. The high-k dielectric layer 54 is formedon the high-k passivation layer 82. The high-k dielectric layer 54includes a high-k dielectric material such as hafnium oxide, lanthanumoxide, aluminum oxide, or the like. The formation of the high-kdielectric layer 54 is performed in the ALD reaction chamber. The high-kpassivation layer 82 is interposed between the high-k dielectric layer54 and the interfacial layer 52 b. Unlike the embodiment shown in FIG.11, the high-k dielectric layer 54 is spaced apart from the interfaciallayer 52 b because of the insertion of the high-k passivation layer 82.The work function metal layer 62 is formed on the high-k dielectriclayer 54. The work function metal layer 62 may include titanium aluminum(TiAl) in accordance with some embodiments.

Reference is still made to FIG. 14. In some embodiments, thesubsequently formed metal layers may include a block layer (not shown),a wetting layer (not shown), and a metal gate electrode 64. The blocklayer may include TiN, and the wetting layer may be a cobalt layer. Themetal gate electrode 64 may include tungsten, a tungsten alloy,aluminium, an aluminium alloy, or the like.

Reference is made to FIG. 15, illustrating a planarization step. Theplanarization step may be, for example, CMP for removing excess portionsof the high-k passivation layer 82, high-k dielectric layer 54, workfunction metal layer 62, and metal gate electrode 64 over the interlayerdielectric layer 42. The interfacial layer 52 b, high-k passivationlayer 82, high-k dielectric layer 54, work function metal layer 62, andmetal gate electrode 64 form the replacement gate stack 92.

The replacement gate stack 92 has the high-k passivation layer 82interposed between the interfacial layer 52 b and the high-k dielectriclayer 54. The interfacial layer 52 b undergoes thermal annealing and TMApretreatment so as to remove the dangling bond thereon, and the high-kpassivation layer 82 prevents germanium out-diffusion from theSi_(1-x)Ge_(x) layer 202. These processes ensure a germanium oxide freeinterfacial layer 52 b and the germanium from the Si_(1-x)Ge_(x) layer202 is confined therewithin. A lower D_(it) can therefore be maintainedat the interface between the interfacial layer 52 b and theSi_(1-x)Ge_(x) layer 202. A lower D_(it) is less likely to flattenon-off switch curve and allows higher electron mobility at the channelregion. Even without the addition of an epitaxial cap on the channelregion, germanium oxide is removed and the remaining free germanium doesnot diffuse out of the Si_(1-x)Ge_(x) layer 202.

Reference is made to FIG. 16. In some embodiments, after the in-situnitridation treatment is performed (see FIG. 10), the high-k passivationlayer 82 is formed on the interfacial layer 52 c which is a siliconoxynitride layer. The formation of the high-k passivation layer 82 isperformed in the ALD reaction chamber. The interfacial layer 52 coverlies the channel region 32, and the high-k passivation layer 82overlies the interfacial layer 52 c. The high-k passivation layer 82conforms to the replacement gate recess 44, in which the sidewalls ofthe spacers 34 and the top surface of the interfacial layer 52 c arecovered up thereby. The high-k passivation layer 82 reacts with theinterfacial layer 52 c. Therefore, the high-k passivation layer 82includes, for example, high-k silicate, germanate, or combinationsthereof in its bottom portion which overlies the interfacial layer 52 c.The concentration of the high-k silicate or germinate in the high-kpassivation layer 82 decreases as a distance from the interfacial layer52 c increases. Examples of high-k materials in the high-k passivationlayer 82 may be Al₂O₃, La₂O₃, Y₃O₃, or combinations thereof. This high-kpassivation layer 82 prevents germanium of the Si_(1-x)Ge_(x) layer 202out-diffusion. A thickness of the high-k passivation layer 82 may rangebetween about 5 and 10 Å.

Reference is made to FIG. 17. A high-k dielectric layer 54 is formed.The high-k dielectric layer 54 includes a high-k dielectric materialsuch as hafnium oxide, lanthanum oxide, aluminum oxide, or the like. Theformation of the high-k dielectric layer 54 is performed in the ALDreaction chamber. The dielectric constant (k-value) of the high-kdielectric material is higher than 3.9, and may be higher than about 7,and sometimes as high as 21 or higher. The high-k dielectric layer 54 isoverlying the high-k passivation layer 82. A work function metal layer62 is formed on the high-k dielectric layer 54. The work function metallayer 62 may include titanium aluminum (TiAl) in accordance with someembodiments. In some embodiments, a barrier layer (not shown) isinterposed between the work function metal layer 62 and the high-kdielectric layer 54. The barrier layer may include TiN, TaN, orcomposite thereof. For example, the barrier layer may include a TiNlayer (the lower part of barrier layer), and a TaN layer (the upper partof barrier layer) over the TiN layer.

Reference is still made to FIG. 17. In some embodiments, thesubsequently formed metal layers may include a block layer (not shown),a wetting layer (not shown), and a metal gate electrode 64. The blocklayer may include TiN, and the wetting layer may be a cobalt layer. Themetal gate electrode 64 may include tungsten, a tungsten alloy,aluminium, an aluminum alloy, or the like.

Reference is made to FIG. 18, illustrating a planarization step. Theplanarization step may be, for example, CMP for removing excess portionsof the high-k passivation layer 82, high-k dielectric layer 54, workfunction metal layer 62, and metal gate electrode 64 over the interlayerdielectric layer 42. The interfacial layer 52 c, high-k passivationlayer 82, high-k dielectric layer 54, work function metal layer 62 andmetal gate electrode 64 form the replacement gate stack 92.

The replacement gate stack 92 has a nitrogenous interfacial layer 52 cand a high-k passivation layer 82 interposed between the interfaciallayer 52 c and the high-k dielectric layer 54. The interfacial layer 52c undergoes thermal annealing and TMA pretreatment so as to remove thedangling bond thereon. The interfacial layer 52 c prevents germaniumout-diffusion from the Si_(1-x)Ge_(x) layer 202, and the high-kpassivation layer 82 is the second barrier against germaniumout-diffusion. Remaining germanium is securely locked in theSi_(1-x)Ge_(x) layer 202 because of the interfacial layer 52 c and thehigh-k passivation layer 82. A lower D_(it) can therefore be maintainedat the interface between the interfacial layer 52 c and theSi_(1-x)Ge_(x) layer 202. A lower D_(it) is less likely to flattenon-off switch curve and allows higher electron mobility at the channelregion 32.

The interfacial layer is firstly annealed to remove germanium oxideafter the interfacial layer formation. Subsequently, TMA pretreatmentthat involves using TMA precursor onto the interfacial layer isperformed. The TMA pretreatment further removes remaining germaniumoxide from the interfacial layer. The interfacial layer then may gothrough nitridation to form a silicon oxynitride layer. Alternatively, ahigh-k passivation layer may be formed on the interfacial layer. Eitherthe nitridation process or the high-k passivation layer preventsgermanium out-diffusion from the Si_(1-x)Ge_(x) layer. Due to theremoval of germanium oxide and germanium out-diffusion blockage, D_(it)can be achieved at the interface between the interfacial layer and thelayer, and therefore the channel region has a higher electron mobility.

In some embodiments, a semiconductor device includes a source region, adrain region, a SiGe channel region, an interfacial layer, a high-kdielectric layer and a gate electrode. The source region and the drainregion are over a substrate. The SiGe channel region is laterallybetween the source region and the drain region. The interfacial layerforms a nitrogen-containing interface with the SiGe channel region. Thehigh-k dielectric layer is over the interfacial layer. The gateelectrode is over the high-k dielectric layer.

In some embodiments, a semiconductor device includes a SiGe layer, asource region, a drain region, a nitrogen-containing interfacial layer,a gate dielectric layer, and a gate electrode. The SiGe layer is over asubstrate. The source region and the drain region are over thesubstrate. At least a portion of the SiGe layer extends laterallybetween the source region and the drain region. The nitrogen-containinginterfacial layer is over the at least a portion of the SiGe layer. Thegate dielectric layer is over the nitrogen-containing interfacial layer.The gate electrode is over the gate dielectric layer.

In some embodiments, a semiconductor device includes a plurality of gatespacers, a silicon oxynitride layer, a high-k dielectric layer and agate electrode. The gate spacers are over a p-type field-effecttransistor (PFET) channel region. The silicon oxynitride layer islaterally between the gate spacers and in contact with the PFET channelregion. The high-k dielectric layer is over the silicon oxynitridelayer. The high-k dielectric layer has a U-shaped profile different froma profile of the silicon oxynitride layer from a cross-sectional view.The gate electrode is over the high-k dielectric layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: forming a silicon germaniumlayer on a substrate; oxidizing a surface layer of the silicon germaniumlayer to form an interfacial layer comprising silicon oxide andgermanium oxide; nitridating the interfacial layer; and forming a metalgate structure over the nitridated interfacial layer.
 2. The method ofclaim 1, wherein the interfacial layer is nitridated using an NH₃plasma.
 3. The method of claim 1 wherein the interfacial layer isnitridated using an N₂ plasma.
 4. The method of clam 1, wherein theinterfacial layer is nitridated using a plasma treatment performed forfive seconds to thirty seconds.
 5. The method of claim 1, wherein theinterfacial layer is nitridated by performing an annealing process in aNH₃ gas.
 6. The method of claim 5, wherein the annealing process isperformed at a temperature in a range from 300° C. to 500° C.
 7. Themethod of claim 1, further comprising: performing an annealing processon the interfacial layer before nitridating the interfacial layer. 8.The method of claim 1, further comprising: performing a trimethylaluminum (TMA) treatment on the interfacial layer before nitridating theinterfacial layer.
 9. The method of claim 1, further comprising:performing an annealing process on the interfacial layer; and afterperforming the annealing process, performing a TMA treatment on theinterfacial layer, wherein the interfacial layer is nitridated afterperforming the TMA treatment.
 10. A method comprising: epitaxiallygrowing a silicon germanium layer on a substrate; performing anoxidation treatment to form an oxide layer of silicon germanium on thesilicon germanium layer; performing a germanium desorption treatment onthe oxide layer to reduce a ratio of germanium oxide to silicon oxide inthe oxide layer; after performing the germanium desorption treatment,nitridating the oxide layer; and forming a metal gate structure over thenitridated oxide layer.
 11. The method of claim 10, wherein thegermanium desorption treatment is a thermal annealing treatment.
 12. Themethod of claim 11, wherein the thermal annealing treatment is performedat a temperature in a range from 500° C. to 900° C.
 13. The method ofclaim 11, wherein the thermal annealing treatment is performed in anoxygen-free ambient.
 14. The method of claim 10, further comprising:performing a TMA treatment on the oxide layer.
 15. The method of claim14, wherein the TMA treatment is performed after performing thegermanium desorption treatment.
 16. A method comprising: forming a layerof semiconductor alloy of a first semiconductor material and a secondsemiconductor material on a substrate; oxidizing a surface portion ofthe layer of semiconductor alloy; treating the oxidized portion of thelayer of semiconductor alloy with trimethyl aluminum (TMA); nitridatingthe oxidized portion of the layer of semiconductor alloy; and afternitridating the oxidized portion of the layer of semiconductor alloy,forming a metal gate structure over the oxidized portion of the layer ofsemiconductor alloy.
 17. The method of claim 16, further comprising:after nitridating the oxidized portion of the layer of semiconductoralloy, forming a high-k silicate layer over the oxidized portion of thelayer of semiconductor alloy.
 18. The method of claim 16, furthercomprising: after nitridating the oxidized portion of the layer ofsemiconductor alloy, forming a high-k germanate layer over the oxidizedportion of the layer of semiconductor alloy.
 19. The method of claim 16,wherein nitridating the oxidized portion of the layer of semiconductoralloy is in-situ performed with treating the oxidized portion of thelayer of semiconductor alloy with TMA.
 20. The method of claim 16,wherein treating the oxidized portion of the layer of semiconductoralloy with TMA is performed at a flow rate of a TMA precursor in a rangefrom 200 sccm to 600 sccm.